In the field of hydrocarbon exploration and production, accurately interpreting array resistivity log data is critical to drilling, formation evaluation, and reservoir characterization. Those of ordinary skill in the art will be familiar with a method for resistivity measurement known as laterolog. Laterolog is a measurement method wherein electric current is forced to flow radially through a downhole formation in a sheet of predetermined thickness, and us commonly used to measure resistivity in hard-rock reservoirs as a method of determining subterranean structural features.
State-of-the-art laterolog devices includes relatively shallow sub-arrays in addition to deeper sub-arrays that are similar to conventional deep and shallow DLL arrays. For example, the “MULL array laterolog tool” produced by Baker Atlas, Houston, Tex., consists of four sub-arrays (multi-level receivers or MLRs) with the following depths of investigation: MLR1—9 inches, MLR2—14 inches, MLR3—18 inches, and MLR4—35-40 inches.
Due to the shallow depth of investigation the measurements of shallow sub-arrays, especially MLR1 in the presently described example, are affected by borehole effects, including, as would be familiar to those of ordinary skill in the art, such factors as the density and effective thickness of intervening materials such as drilling fluid, mud cake, casing, cement, and rock matrix between the resistivity sensor arrays and the formation under investigation. Such borehole effects are especially pronounced in very conductive muds and in very large boreholes.
Borehole effects can be severe, such that conventional borehole corrections based just on borehole size, mud conductivity, and fixed value of tool eccentricity (typically used for dual laterolog (DLL) measurements) become ineffective. In addition, the MLR1 measurement, being relatively shallow, becomes very sensitive to radial position of the tool in hole, i.e., tool eccentricity. Often the tool eccentricity arbitrarily changes while logging, especially in the presence of borehole washouts, so the use of fixed eccentricity values does not provide adequate borehole correction.